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The 17th Pokémon movie was revealed in the Oha Suta episode of December 13th 2013 and CoroCoro Issue released December 14th 2013 and features Xerneas & Yveltal and is due for release in Japan in July 19th 2014. It also comes with an attached Pikachu short called Pikachu, What Kind of Keys Are These?.

Not much is known about this movie, just that it features Xerneas & Yveltal as well as Mega Evolutions. On February 11th 2014, it was confirmed that the Pokémon Diancie would star in the movie and the movie was re-named to show its inclusion

Summary

In the Diamond Kingdom, Diancie is hiding from her three bodyguard Carbinks. A fourth Carbink, the elder Carbink Daii, appears and scolds Diancie. He shows her the Sacred Diamond, and says it will soon begin to lose its power. Diancie says he is only exaggerating, but he insists he is not. He says only she can create a new Sacred Diamond, and she seems doubtful of herself but tries anyway. The diamond is formed bur quickly dissolves to everyone's disappointment. Daii says they should go find Xerneas who can give Diancie more power using Fairy Aura. He remembers a time when a forest was having all the life sucked from it, and Xerneas saved him and other Pokemon nearby from the destruction. Diancie looks forward to going outside, and is moved by the sight when they finally emerge from the caves of the Diamond Kingdom.

During the opening credits, Diantha and Wikstrom battle with Mega Gardevoir and Mega Scizor. Ash and the group are introduced and Ash battles Astrid's Male Pyroar, Female Meowstic and Mega Absol with Froakie, Pikachu and Hawlucha. During the battle, Diancie appears in town being chased by Marilyn Flame and Ninja Riot. Bonnie proposes to Astrid for Clemont. Ash realizes something is odd and runs towards the attacks he can hear, protecting Diancie and allowing her to get away safely.

Ash, Clemont, Serena, and Bonnie introduce themselves to Diancie, who they find very polite and like a princess. They discover from the Pokedex she should be able to make diamonds. She is very polite ("like a princess!") but unworldly, not knowing what a thief is until they tell her. They go to get dinner together, and Team Rocket successfully captures Diancie in a sack. They take her into a clock tower and demand she make them diamonds, which she agrees to do happily. After producing many diamonds, Team Rocket is too happy and distracted by their riches to care as she simply attempts to leave. The door won't open until Miris Steel appears to open it for her, helping her to escape.

Diancie is reunited with Ash and friends, and they decide to go together to find Xerneas with Diancie. Diancie mentions about now the thieves will be very disappointed and explains the diamonds she makes vanish. Indeed, the diamonds Team Rocket was swimming in vanish at that time.

Diancie and Ash and friends head out by bus first, then ship, following the way Diancie can sense due to Fairy Aura. At night, Diancie and Ash see a shooting star after she yet again fails to produce a Sacred Diamond. Ash says you have to wish on a shooting star before it vanishes, and when another appears, he wishes for Diancie to be able to make diamonds. Serena watches them from her tent with a smile.

On the ship, Diancie tries again to make a diamond to show her friends. She makes one and there is a little spark when this one is made. Bonnie asks if she can keep it and puts it into her bag to keep, saying she's happy even if it will vanish.

They go shopping, which Diancie has never done before, so Serena and Bonnie take her to try on various clothing. After the fashion show, Diancie sees her three bodyguards in the mall looking for her and panics. Thinking she is running from them because they are bad, Ash and friends try to help. She is on a higher floor being chased, so Clemont pulls an extendable robot ladder out of his backpack and Ash uses it to grab Diancie. It doesn't explode, either, and works! They make their getaway.

Once outside, they are attacked again by Marilyn and Ninja. While Ash holds them off, Serena finds a Rhyhorn and gets Bonnie, Diancie and Clemont (clinging to its butt) out by racing off. They even partially run on a Rhyhorn racetrack before veering off into the forest. When Greninja wraps its tongue around Diancie to drag her off, Bonnie orders Dedenne to use Nuzzle to make it let go.

Eventually they are saved by the Steels again and able to get away. Marilyn and Ninja are taunted a little by Mirris. The Carbinks catch up again to Diancie and she explains they are her bodyguards, and she actually is their princess. The Carbinks tell her the Sacred Diamond has died. They go through the Diamond Kingdom, which is actually many different diamond caves connected by multiple tunnels, all kept lit and alive by the Sacred Diamond. Everything inside is now dissolving and collapsing due to it having faded. Diancie thinks herself a failure and begins to cry, but they convince her it will be okay. They come through to the Oruaas Forest, where Diancie senses the Fairy Aura very strongly. Daii explains this is the same forest that was destroyed before by Yveltal, who was stopped by Xerneas before cocooning and going into a long sleep.
Diancie goes after Xerneas, and they catch one glimpse before it runs. Diancie keeps chasing it, tripping and falling down often, but finally she catches up. Xerneas says it knows why Diancie has come and tells her the power is within herself. Xerneas lets the Fairy Aura bathe over everything nearby, including Diancie and her Carbinks, and even Ash and friends can feel its power.

Once Xerneas leaves, before Diancie can test if she now has the power to create a Sacred Diamond, the Steels arrive to capture Diancie. They reveal they'd only helped her before because it was pointless to try to capture her before she received the Fairy Aura power. Ninja and Marilyn and the Steels all fight one another to catch Diancie, eventually bringing the fight to an underground pool. Everytime they step in the water, it reverberates down to the cocoon in the bottom of the pool, which begins to awaken. The more they battle, the more the cocoon begins to unravel.

They finally realize something is wrong, and everyone gets out of the pool just as Yveltal awakens and flies out. It begins to destroy and petrify parts of the forest, including the Carbinks and Team Rocket, who was after Diancie as well. It also petrifies Delphox and Greninja. While trying to stop Yveltal Ash nearly falls off a cliff, and Clemont, Serena and Bonnie pull him back up.

Diancie blocks the way to protect Ash and friends, and remembers Xerneas saying the power is within herself. There is a spark in Bonnie's bag, and Diancie Mega Evolves, creating a diamond large enough to block Yveltal's destructive attack, protecting her friends. The diamond doesn't break up or dissolve, it is a true diamond.
The Steels try to stop Yveltal in their ship, shooting missiles at it, but Yveltal flies over and petrifies them and their Chesnaught, and they crash into the lake. Marilyn and Ninja, upset over Delphox and Greninja, stand together and hug one another as they too are frozen. Xerneas appears then, and its mere appearance seems to calm Yveltal, which flies away.

The danger seems over, but the forest and Pokemon and people are still petrified. Pikachu is weak in Ash's arms after fighting Yveltal, and begins to turn stiff as well. They realize its tail was just barely hit by one of Yveltal's destructive beams. Ash can only look on in horror as Pikachu freezes completely. He cries over his Pokemon, as does Serena, Bonnie and Clemont, but Xerneas quickly begins to send out a healing power through the forest, unfreezing Pikachu and the others, and reviving everything Yveltal destroyed.

As they look on, Xerneas begins to transform into a tree. They ask if this means it will die, but it reassures them it is just the start of a new life. They return to the Diamond Kingdom, where Diancie mega evolves again and creates a new Sacred Diamond, reviving the whole kingdom and lighting it up again. Bonnie reaches into her bag, finding the diamond Diancie first made is still there. They are surprised that it didn't disappear despite being made before Diancie received the Fairy Aura power from Xerneas. Bonnie says it is due to their friendship, and returns it to Diancie as it is an important thing to her, her first real diamond. When the camera closes up on it, the mega symbol is glowing inside of it. It has become a mega stone.

During the credits, Ash and company continue traveling, Diancie continues to make diamonds with her Carbinks, and the Steels have opened up a new chocolate shop together. Ninja Riot proposes to Marilyn Flame (she accepts). The end!